Comprehensive Study Guide: Cell Division, Genetics, DNA, Evolution, and Biotechnology

Cell Division and the Cell Cycle

Interphase and Its Stages

Interphase is the phase of the cell cycle during which the cell grows, replicates its DNA, and prepares for division. It consists of three main stages:

• G1 Phase (Gap 1): Cell grows and carries out normal metabolic functions. Organelles are duplicated.

• S Phase (Synthesis): DNA is replicated, resulting in two sister chromatids for each chromosome.

• G2 Phase (Gap 2): Cell continues to grow and prepares for mitosis. Checks for DNA errors and repairs them.

Mitosis Stages

Mitosis is the process by which a eukaryotic cell divides its nucleus and contents. The stages are:

• Prophase: Chromatin condenses into visible chromosomes; spindle fibers form; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equatorial plate.

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear envelopes reform around each set of chromosomes; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Chromosomes, Chromatids, and Chromatin

• Chromosome: A single, long DNA molecule with associated proteins, visible during cell division.

• Chromatid: One of two identical halves of a replicated chromosome.

• Chromatin: The complex of DNA and proteins (histones) in the nucleus; less condensed than chromosomes.

Haploid, Diploid, and Polyploidy

• Haploid (n): One set of chromosomes (e.g., gametes).

• Diploid (2n): Two sets of chromosomes (e.g., somatic cells).

• Polyploidy: More than two sets of chromosomes; common in plants.

Cell Cycle Checkpoints and Cancer

Checkpoints ensure the cell only proceeds to the next stage if conditions are favorable. Failure can lead to uncontrolled cell division (cancer).

• Oncogenes: Mutated genes that promote cell division.

• Tumor Suppressor Genes: Genes that inhibit cell division; mutations can lead to cancer.

Results of Mitosis and Meiosis

• Mitosis: Produces two genetically identical diploid cells.

• Meiosis: Produces four genetically unique haploid gametes.

Why Gametes Are Haploid

Gametes must be haploid so that upon fertilization, the resulting zygote is diploid, restoring the species' chromosome number.

Genetics and Inheritance

Heterozygous vs Homozygous

• Homozygous: Two identical alleles for a gene (e.g., AA or aa).

• Heterozygous: Two different alleles for a gene (e.g., Aa).

Genes, Alleles, Genotype, and Phenotype

• Gene: A segment of DNA that codes for a protein.

• Allele: Different forms of a gene.

• Genotype: The genetic makeup (e.g., AA, Aa, aa).

• Phenotype: Observable traits (e.g., flower color).

Dominant and Recessive Alleles

• Dominant: Expressed when at least one allele is present.

• Recessive: Expressed only when both alleles are recessive.

Incomplete Dominance and Codominance

• Incomplete Dominance: Heterozygote shows an intermediate phenotype (e.g., red x white = pink flowers).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

Punnett Squares

Punnett squares are used to predict the probability of offspring genotypes and phenotypes.

• 2x2: Monohybrid cross (one gene).

• 4x4: Dihybrid cross (two genes).

Polygenic Traits

Traits controlled by multiple genes, resulting in a range of phenotypes (e.g., human skin color).

Gene Linkage

Genes located close together on the same chromosome tend to be inherited together.

Sex Chromosomes and X-Linked Traits

• Sex Chromosomes: X and Y chromosomes determine biological sex.

• X-Linked Mutations: Males are more affected because they have only one X chromosome.

Pedigree Analysis

Pedigrees are diagrams that show inheritance patterns in families. Used to determine genotypes and predict inheritance of traits.

Chromosomal Mutations

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Inversion: Reversal of a chromosome segment.

• Translocation: Segment moves to a nonhomologous chromosome.

Dosage Compensation and X Inactivation

Mechanisms to balance gene expression between sexes. In females, one X chromosome is inactivated (Barr body).

Why Different Chromosome Numbers Are Problematic

Abnormal chromosome numbers can disrupt gene dosage, leading to developmental disorders (e.g., Down syndrome).

DNA, Gene Expression, and Biotechnology

DNA Replication and the Semiconservative Model

• DNA Replication: The process of copying DNA before cell division.

• Semiconservative Model: Each new DNA molecule consists of one old and one new strand.

Heterochromatin vs Euchromatin

• Heterochromatin: Densely packed, transcriptionally inactive DNA.

• Euchromatin: Loosely packed, transcriptionally active DNA.

Restriction Enzymes and DNA Ligase

• Restriction Enzymes: Cut DNA at specific sequences.

• DNA Ligase: Joins DNA fragments together.

PCR (Polymerase Chain Reaction)

PCR amplifies specific DNA sequences using:

• Denaturation: DNA strands are separated by heating.

• Annealing: Primers bind to target sequences.

• Extension: DNA polymerase synthesizes new DNA.

Gene Cloning

Involves inserting a gene of interest into a vector, introducing it into a host, and producing copies.

Applications of Gene Cloning

• Production of insulin and growth hormone.

• Genetically Modified Organisms (GMOs).

Gene Therapy

• Somatic Cell Therapy: Targets body cells; changes are not inherited.

• Germline Therapy: Targets gametes; changes are heritable.

Electrophoresis Gel Interpretation

Used to separate DNA fragments by size. Can determine relationships (e.g., paternity testing).

SNPs (Single Nucleotide Polymorphisms)

Single base-pair variations in the genome; useful as genetic markers.

Pseudogenes

Nonfunctional gene sequences resembling functional genes.

Evolution and Population Genetics

Charles Darwin and the Four Postulates of Evolution

• Variation exists among individuals.

• Some variation is heritable.

• More offspring are produced than can survive.

• Individuals with advantageous traits survive and reproduce more.

Evolution Defined

Evolution is the change in allele or trait frequency in a population over time.

Artificial Selection

Humans select for desirable traits (e.g., dog breeding).

Species Concept

A species is a group of organisms that can interbreed and produce fertile offspring.

Evidence for Evolution

• Cellular Evidence: Similarities in cell structure and function.

• Biogeography: Geographic distribution of species.

• Homologous Structures: Anatomical similarities due to common ancestry.

Hardy-Weinberg Equilibrium

Describes a non-evolving population. Useful as a null hypothesis in population genetics.

Equation:

$p^2 + 2pq + q^2 = 1$

$p + q = 1$

Agents of Evolutionary Change

• Mutation

• Gene flow (migration)

• Genetic drift

• Non-random mating

• Natural selection

Natural Selection vs Evolution

• Natural Selection: Mechanism by which advantageous traits become more common.

• Evolution: The overall change in allele frequencies.

Genotype, Phenotype, and Fitness

Genotype determines phenotype; phenotype affects fitness (reproductive success).

Speciation

Process by which one species splits into two due to reproductive isolation.

Parsimony and Family Trees

Parsimony principle: the simplest explanation (fewest evolutionary changes) is preferred when constructing phylogenetic trees.

Additional Topics and Applications

Why Recessive Alleles Are Hard to Remove

Recessive alleles can persist in heterozygotes, making them difficult to eliminate from a population.

Dogon of Mali and Breast Cancer

The Dogon people have a lower incidence of breast cancer, possibly due to genetic or environmental factors (e.g., diet, reproductive patterns).

Additional info: The study of population genetics and epidemiology can help explain such differences.

Term	Definition	Example/Application
Haploid	One set of chromosomes	Gametes (sperm, egg)
Diploid	Two sets of chromosomes	Somatic cells
Polyploidy	More than two sets of chromosomes	Wheat (hexaploid)
Oncogene	Mutated gene promoting cell division	Ras gene
Tumor Suppressor	Gene inhibiting cell division	p53 gene
Restriction Enzyme	Enzyme that cuts DNA at specific sites	EcoRI
DNA Ligase	Enzyme that joins DNA fragments	Used in cloning
SNP	Single nucleotide polymorphism	Genetic marker